Elan +2 Ignition Timing

[JonB]

What's your engine configuration?

[chickenstock10k]

Federal Elan +2, though that doesn't impact the advance curve at the dizzy

[2cams70]

That stuff in the Miles Wilkins book is a shemozzle. Be aware there's advance and RPM at the crankshaft and also at the distributor. A distributor is usually bench tested for calibration hence the figures are often at the distributor and not the crankshaft. The distributor runs at half crankshaft speed. Therefore for both advance and RPM if it's measured at the distributor it's X 2 at the crankshaft. Mechanical advance usually kicks in above 1,000 crankshaft RPM. It's not clear in Miles Wilkins where the reference is - distributor or crankshaft.

[JonB]

I agree that the description could be more lucid, but the graph on page 169 seems pretty clear. The axes are labelled "Crankshaft RPM" and "Degrees advance at crankshaft", and there is a curve for each combination of distributor and engine type / carburettor. This is what I use for my timing: for the UK ("domestic") Big Valve engine running Dellortos and a 41189A distributor, it's 12 degrees at tick over.

However - full disclosure here - I'm not an expert, so must rely on other people like Miles for insights.

[69S4]

I?ve always found the information in MW?s tc book pretty clear but even if it wasn?t there?s not much you can do with it anyway without going to considerable time and / or expense. You can set the static timing (give or take a degree or two) and maybe get a rough idea of the advance curve with a timing light but the agonising over the finer points of how many degrees at what rpm is beyond my home capacity anyway.

And who knows what the actual optimum advance curve for any particular engine is anyway, what with changes in fuel formulation, engine wear / rebuilds etc since the 60?s. Yes, with time on a rolling road and maybe something like a 123 programmable distributor you could get it right but that?s a long way from what many of us are willing to do. I?m sure there are some who?ve done it though. Maybe they could chip in with some insight as to how close the Wilkins / workshop manual data is to the optimum.

[richardy8496]

Thanks guys for all your input. I seem to have started a highly technical discussion when all I intended was to tune the engine so that it ran sweetly without spitting and having a nasty flat spot. I've now set the timing at 12 degrees BTDC at tickover (+/- 1000 RPM) and then balanced and adjusted the Webers - it's a lot better, runs sweetly when not under load but still has a flat spot on the road when given full throttle at low revs. I've seen a lot of discussion about idle jet sizes - mine are 45F9's, which appear to be the right ones and presumably have worked in the previous 120,000 miles the car has done. Anything I have missed? Failing that, does anyone know a good place in the Sussex/Kent area that can get the engine sorted, preferably on a rolling road so that load can be applied?

[2cams70]

In the tables MW has for example with the 41189 distributor advance occurring up to 5,000 distributor RPM (i.e 10,000 engine RPM). This cannot be. In the charts we have two curves for the 41189 distributor. One for the Big Valve and a slightly different one for the SE. This also cannot be. One distributor identification number must equal only one advance curve.

Looking at the Ford manual for the Twin Cam SE spec. (as used in Escort Twin Cam) - i.e 41189 distributor we have two curves too but they represent the upper and lower tolerance boundaries for the curve and not different applications. In the Ford manual advance should begin at 400-600 distributor RPM (i.e 800 to 1,200 crankshaft RPM and not 2,000RPM as per MW).

[rgh0]

Federal Elan +2, though that doesn't impact the advance curve at the dizzy

Yes a stromberg set up is very different from a Weber / Dellorto

[chickenstock10k]

My point is that even though our engines are different the dizzy is the same and so my configuration isn?t all that helpful.

[JohnCh]

In the tables MW has for example with the 41189 distributor advance occurring up to 5,000 distributor RPM (i.e 10,000 engine RPM). This cannot be. In the charts we have two curves for the 41189 distributor. One for the Big Valve and a slightly different one for the SE. This also cannot be. One distributor identification number must equal only one advance curve.

The numbers in his books are total advance at the crank, including static for each engine/distributor combination. If you look at the two curves for the same distributor and then look at the static settings for the two engines using that distributor, you will see that the advance difference at each rpm is the same as the static advance difference. i.e. for the 41189A, the Big Valve has 2 deg more static advance than the SE, and it's curve is 2 deg higher at every rpm, so the distributor's native advance curves are the same.

-John

[2cams70]

The numbers in his books are total advance at the crank, including static for each engine/distributor combination. If you look at the two curves for the same distributor and then look at the static settings for the two engines using that distributor, you will see that the advance difference at each rpm is the same as the static advance difference. i.e. for the 41189A, the Big Valve has 2 deg more static advance than the SE, and it's curve is 2 deg higher at every rpm, so the distributor's native advance curves are the same.

Now accepted that the 2 degree difference in the static timing gives a 2 degree difference in total advance across the range between SE and Big valve engines. The MW charts are still wrong by a huge margin however. Whilst the vertical axis for advance appears correct the horizontal axis for crankshaft RPM appears to be out by a factor of two. Divide the horizontal axis numbers by 2 and you get something that is approximately correct. I can only speak of the 41189 distributor as I don't have information about the others. Like I said the ignition timing information in MW is a shemozzle. See attached information from the Ford manual for the SE Twin Cam. The upper and lower lines represent the upper and lower tolerance boundaries. Ford figures for the ignition are at the distributor for both advance timing and RPM. Timing advance shown does not include the static figure.

[JohnCh]

I just looked at the workshop manual and although Wilkins data essentially matches for the 40953 (he is 0.5 deg higher at 5000 and 6000 rpm) it confirms what you show below for the 41189. Wilkins does write "Please note that in the manuals, Lotus get confused between crankshaft rpm and distributor rpm." If you double the rpm speeds in the manual for the 41189, you get what Wilkins publishes as the curve. Given the 40953 increases advance as it approaches 6500 rpm, it seems a little odd that the 41189 hits max advance at just 2500 rpm. The other distributors he details also peak at 5000 rpm or higher, but I don't see any of those in my workshop manual to compare. I wonder if Miles is correct and the manuals are wrong for that specific distributor?

-John

[2cams70]

It seems to be that the higher the state of engine tune the less the ignition advance required at high RPM. On that basis I'd suggest that the Ford/Lotus manual is correct for the 41189 distributer and MW is wrong. Refer attached charts for the crossflow engine in both standard and GT stages of tune. For the GT we see that the secondary stage of advance from around 2,200 RPM (crankshaft) to 5,300 RPM (crankshaft) is much less aggressive than the standard tune. The Twin Cam 41189 takes this even further with only primary advance up to 2,400 RPM (crankshaft) and no second stage advance beyond that. Your guess is as good as mine about the other distributors though!!